Method of making rolled ferrosilicon alloys



States 2,802,761 Patented Aug. 13, 1957 METHOD OF MAKING ROLLED FERROSILICON ALLOYS Johann Diedrich Fast, Eindhoven, Netherlands, assignor,

by mesne assignments, to North American Philips Company, Inc, New York, N. Y., a corporation of Dela- Ware No Drawing. Application September 22, 1955, Serial No. 536,042

Claims priority, application Netherlands September 23, 1954 7 Claims. (Cl. 148-113) The object of the invention is to provide ferrosilicon alloys of improved magnetic properties and containing 2 to 5% silicon. It is known that rolled ferrosilicon has its maximum permeability in the rolling direction if the crystals have a preferred orientation indicated by the symbols (110) [100]. These symbols mean that the so-callecl (110) crystal faces extend substantially parallel to the surface of the rolled material and furthermore that the [100] directions are substantially parallel to the rolling direction.

Ferrosilicon alloys having said texture are obtained by subjecting the alloys to cold-rolling approximately 800 C. and an interim annealing operation at approximately 900 C. and, if required, by repeating these treatments several times in succession. Since cold-rolling, more particularly of large blocks imposes stringent requirements on the required roll-stands, it has furthermore been proposed to make blocks or bars of said material by hotrolling from a temperature of approximately 1100 C. to about to 7 times the desired thickness and subsequently cold-rolling the material thus obtained, as stated above, to its desired final thickness. The cold-rolled material is finally annealed at a comparatively high temperature, that is to say approximately 1100 C., thus creating the aforesaid preferred orientation and firing out undesired impurities. In this case the terms cold-rolling and hot-rolling are to be understood to mean rolling below or above the recrystallisation temperature respectively.

Apart from the rolling operation and heat treatment care has been bestown on the composition of ferrosilicon alloys, inter alia on the influence of impurities. Thus, for example, the influence of phosphorus and arsenic on the magnetic properties of said type of alloys has been investigated and furthermore it has been proposed to add aluminium to a maximum of 5 by weight to ferrosilicon alloys. Serious consideration has furthermore been given to the influence of nitrogen. From several publications it is found that for obtaining good magnetic properties the presence of nitrogen in ferrosilicon alloys, should be avoided, since for example, no texture could be obtained with a nitrogen content of approximately 0.02% by weight in ferrosilicon alloys.

Furthermore it has been pointed out that when annealing ferrosilicon alloys in pure hydrogen the initial permeability is considerably higher than if the hydrogen contains 0.3 by weight of nitrogen.

Researches preceding the invention revealed that in very pure ferrosilicon, that is to say in alloys wherein no elements other than silicon and iron could be found, no (110) [100] texture is obtainable by cold-rolling combined with-interim annealing operations. In carrying out further tests measured quantities, each time of one element, were added to the ferrosilicon alloys. Completely in contradistinction to What might be expected from the known literature it was found that the (110) [100] texture is readily obtained by introducing nitrogen into pure ferrosilicon alloys, provided given conditions be fulfilled. Continued investigations showed that similar results are obtainedunder the same circumstancesif nitrogen is introduced into technical ferrosilicon alloys. The last-mentioned alloys can even be given magnetic properties better than those hitherto known.

In accordance with the invention, ferrosilicon alloys having a silicon content between 2% and 5% by weight are subjected in order to secure good magnetic properties, to cold-rolling, if desired preceded by hot-rolling, and to one or more intermediate annealing operations, the invention being characterized in that the alloy, prior to reduc ing it to its final thickness by cold-rolling, is heated above 400 C. in a streaming gas mixture containing such a nitrogen compound of such concentration as to dissolve nitrogen to a quantity of at least 0.01% by weight in the alloy, without the formation of iron nitrite or austenite, the nitrogen, after cold-rolling the alloy to its final thickness, being expelled by annealing the alloy in a hydrogencontaining atmosphere free from nitrogen, which atmosphere does not contain any substances having an oxidizing effect on the ferrosilicon.

The effect in accordance with the invention is not secured if the alloy is heated in a streaming gas mixture exclusively containing molecular nitrogen as a nitrogen compound, since said gas is slightly dissolved by ferrosilicon alloys. Contrary thereto, excellent results are obtained by heating the alloy in a streaming gas mixture of ammonia and hydrogen. An explanation of the fact why NH3 is so much more active than N2 has been given in Philips Technical Review of January 1948, pages 30 and 31.

The concentration of the nitrogen-containing compounds in the gas-mixture should not be so high as to form iron nitrides or austenite during the heating operation. If ammonia is used as a'nitrogen-containing compound the data for a correct concentration may be found in a publication by Lehrer in Zeitschrift fiir Elektro chemie, 36, 1930, page 383. These data hold for nonalloyed iron, it is true, but may also be used in the present case. In the table below, stating said data, the NHs concentrations are listed which should not be exceeded at a given temperature of the gas mixture.

Temperature in O.

700 750 and higher less than 2 The remainder of the gas is hydrogen. In the presence of further gases the aforesaid percentages do not refer to the total gas volume but to the sum of the volumes of hydrogen and ammonia. The presence of oxygen-containing compounds such as, for example, water should generally be avoided, since these compounds may give rise to the production of a SiOz-c0ntaining film difiiculty pervious to nitrogen on the surface of the alloy. However, a low water-vapour pressure of, say, 1 mm. Hg is permissible, notably if ammonium chloride is added to the gas, for example several tens of mg. per litre of the gas. As a matter of fact, said substance prevents the formation of a silicon dioxide-containing film on the alloy. Excellent results are obtained if the nitrogen content of the alloy has a value of 0.12 to 0.01% by weight. By analysis the man of the art can determine the concen- 3 tration of the nitrogen-containing compound of the gas mixture, the temperature at which it should be heated, the rate of flow and the heating-time required to obtain said nitrogen concentration of the alloy.

In accordance with the statement of invention it is necessary for obtaining good magnetic properties that thealloy enriched in nitrogen should finally be so heated as to completely expel the nitrogen. It has been found that this can only be achieved by annealing in a reducing hydrogen-containing atmosphere, the water-vapour pressure of which is lower than 0.001 mm. Hg.

The annealing operation to expel nitrogen may be effected within suitable temperature ranges, for example between 1100 C. and 1300 C. or between 900 C. and 1000 'It may alternatively occur in two stages, for example by first heating the material at 900 C. to 1000 C. and subsequently at 1100 C. to 1300" C. The last-mentioned method confers excellent properties on the material but proved very costly in practice. The same excellent results are obtained more economically by heatingthe, alloy, in a turther form of the invention, first at 500 C. to 800 C. and subsequently at 900 C. to 1000 C.

EXAMPLE A bar of pure ferrosilicon containing 2.8% by weight of Si was rolled to a thickness of 3.0 mm. at 850 C. in a hydrogen current and subsequently heated for two hours at a temperature of 550 C. in a streaming gas mixture containing 87% by volume of hydrogen and 13% by volume of ammonia, the rate of flow being 1 litre per minute, 0.04 g. ammonium chloride per litre having been added to the gas mixture. Subsequently it was subjected to an intermediate annealing operation at 760 C. for 24 hours in a streaming gas mixture containing 75% by volume of nitrogen and 25% by volume of hydrogen. After that the nitrogen content of the alloy was 0.076% by weight, In five times the material was subsequently reduced by cold-rolling to a thickness of 0.6 mm. and again annealed for two hours at 900 C., the gas mixture consisting of 75% by volume of nitrogen and 25% by volume of hydrogen. After this operation the nitrogencontent of the alloy still was 0.076% by weight. The bar was again cold-rolled and its thickness reduced to 0.3 mm. The final annealing operation was carried out in streaming pure hydrogen (water-vapour pressure 10- mm. mercury) for four hours at 600 C. and then for four hours at 950 C. After this annealing operation the alloy consisted of crystals 10 mm. to 30 mm. in diameter.

The permeability measured in a direction parallel to the rolling direction at a field strength of 10 oersteds was 1850 gauss/oersted. When carrying out the test with the same starting material, but without the alloy being heated in the ammonia-containing gas, the crystals were approximately 0.1 mm. to 0.3 mm. in diameter. The permeability in the rolling direction at a field strength of 10 oersteds was now only 1470 gauss/oersted.

What is claimed is:

1. In the method of making a ferro-silicon alloy having a silicon content of between about 2 and 5% by weight and having good magnetic properties, by subjecting the alloy to cold rolling and at least one annealing operation, the steps comprising, heating the alloy to a temperature above about 400 C. in a streaming gas mixture containing a nitrogen compound in a concentration sufiicient to dissolve at least 0.01% by weight of nitrogen in the alloy without formation of iron nitrides and austenite, coldrolling the alloy to its final thickness and then expelling the nitrogen from the alloy by annealing the alloy in a hydrogen containing atmosphere free from nitrogen and substances having an oxidizing effect on the ferrosilicon alloy.

2. The method of claim 1 in which the alloy is subjected to hot-rolling combined with at least one interim annealing operation before cold-rolling.

3. The method of claim 1 in which the streaming nitrogen compound containing gas mixture is a mixture of ammonia and hydrogen.

4. The method of claim 3, in which the streaming gas mixture contains ammonium chloride.

5. The method of claim 1 in which the heating operation in the nitrogen compound containing atmosphere is carried out at a temperature, of between 500 C. and 600 C.

6. The method of claim 1 in which in the annealing operation used to expel the nitrogen, the alloy is first heated at 500 C. to 800 C. and subsequently at 900 C. to 1000 C.

7. The ferrosilicon alloy sheet made by the process of claim 1.

Cole et al. May 16, 1939 Jackson et al. Jan. 15, 1952 

1. IN THE METHOD OF MAKING A FERROR-SILICON ALLOY HAVING A SILICON CONTENT OF BETWEEN ABOUT 2 AND 5% BY WEIGHT AND HAVING A GOOD MAGNETIC PROPERTIES, BY SUBJECTING THE ALLOY TO COLD ROOLING AND AT LEAST ONE ANNEALING OPERATION, THE STEPS COMPRISING, HEATING THE ALLOY TO A TEMPERATURE ABOVE ABOUT 400*C. IN A STREAMING GAS MIXTURE CONTAINING A NITROGEN COMPOUND IN A CONCENTRATION SUFFICIENT TO DISSOLVE AT LEAST 0.01% BY WEIGHT OF NITROGEN IN THE ALLOY WITHOUT FORMATION OF IRON NITRIDES AND AUSTENITE, COLDROLLING THE ALLOY TO ITS FINAL THICKNESS AND THE EXPELLING THE NITROGEN FROM THE ALLOY BY ANNEALING THE ALLOY IN A HYDROGEN CONTAINING ATMOSPHERE FREE FROM NITROGEN AND SUBSTANCES HAVING AN OXIDIZING EFFECT ON THE FERROSILICON ALLOY. 